1. Field of the Invention
The present invention generally relates to a method and an apparatus for generate a spread-spectrum clock, in particular to an electronic device using a spread-spectrum clock.
2. Description of the Related Art
Clock circuit is one of the important components for most electronic devices, which will influence their overall system performance. However, the clock signal will generate strong electromagnetic interference (EMI) on its frequency. Thus, in the industry, a threshold value will be defined as the upper limit of the electromagnetic interference generated by the clock signal. The current technology usually uses the spread-spectrum clock to dynamically change the frequency of the clock signal to distribute the energy of the clock signal in order to conform to the EMI requirement.
The conventional spread spectrum controller is composed of a phase locked loop with a loop filter, which is a closed-loop type circuit structure. When the spread spectrum controller is provided to control the phase locked loop to execute the spread spectrum process for the clock signal, it is not easy to know the timing for each spread spectrum command starting to affect the phase locked loop. Besides, it is also not easy to make sure how much clock phase offset will be generated for each spread spectrum command. However, the above information is relatively important for a circuit system with constant throughput. Accordingly, it is necessary to develop a new spread spectrum clock generating technology to solve the problems.